I am not sure it is intended to be persistent, unless you use restore previous session rather than a home page. Maybe there is some preference that can be set. Make sure you exit from firefox using Firefox button -> exit

It used to be persistent in previous versions (don't know exactly when it changed). Of course I exit normally with [X] or Exit. Restoring a previous session is enabled and seems to work. But a new browser window (when there are already some open) again is without sidebar, even if all the other windows have the sidebar enabled.

Is that a pop-up window that has the sidebar missing?

If it only happens in one window then close that window or tear-off the tabs to a new window to see if that makes the sidebar persist.

It's not a pop-up window, just a normal browser after clicking on the FF-icon on the desktop or starting "firefox.exe" on the command line. No, closing and reopeing it doesn't help.

Can you tell me where this setting should be saved? Then I can check if it is not stored at all or if it is not active while opening an additional browser window.

If I have a browser with bookmarks sidebar and I do a "open link in new window" then the new browser window does have the bookmarks sidebar. It's only a completely new browser that has no sidebar.

Such data is stored in the file localstore.rdf in the profile folder.

• http://kb.mozillazine.org/localstore.rdf
• http://kb.mozillazine.org/Corrupt_localstore.rdf

OK, but what is the name of the setting? I have already tried the fixing as mentioned in the first post. The xml syntax looks well formed.

-> Tap ALT key or press F10 to temporarily show the Menu bar

• go to View Menu -> Sidebar -> select Bookmarks

Check and tell if its working.

Yes, this works, as written in my first post. That was never the problem but the persistence across browser windows if I open a new one.

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.com/kb/Safe+Mode

Doesn't help, after restarting FF and opening a new browser window the sidebar is gone again (not in the restored windows, only in the new one).

I think I finally got it to work. I have a second computer (same setup) where it works. So I compared the localstore.rdf files and found the problem.

 <RDF:Description RDF:about="chrome://browser/content/browser.xul#sidebar-box"
                  sidebarcommand="viewBookmarksSidebar"
                  width=""
                  src="chrome://browser/content/bookmarks/bookmarksPanel.xul" />

The sidebar was enabled but the width was empty so in effect disabled. Also closing a browser with the sidebar showing didn't change this value. After manually entering a value I can now open a new browser window with sidebar enabled.

Glad it works, congratulations on discovering the solution.